Physical activity, sleep duration, and cognitive ageing – Authors’ reply
Authors: Mikaela Bloomberg, Laura Brocklebank, Mark Hamer, Andrew Steptoe
Journal: The Lancet Healthy Longevity (2023)

We thank Wenming Shi for his letter, which suggests important areas for future research on this topic. As the number of hours in a day is finite, an increase in sedentary behaviour necessitates a decrease in sleep or physical activity. Because our study examines time spent sleeping and doing physical activity, adjusting for time spent being sedentary, which can be inferred from sleep and physical activity duration, is redundant. The isotemporal substitution methods used in the paper cited by Shi allow sleep, physical activity, and sedentary behaviour to be examined simultaneously,1Wei J Hou R Xie L et al.Sleep, sedentary activity, physical activity, and cognitive function among older adults: the National Health and Nutrition Examination Survey, 2011–14.J Sci Med Sport. 2021; 24: 189-194Summary Full Text Full Text PDF PubMed Scopus (0) Google Scholar but require objective measurement of 24-hour movement behaviours, and availability of longitudinal data is limited. Furthermore, associations between sedentary behaviour and cognitive function are complex; some sedentary behaviours (eg, puzzles and computer use) have been associated with better cognitive performance,2Ferreira N Owen A Mohan A Corbett A Ballard C Associations between cognitively stimulating leisure activities, cognitive function, and age-related cognitive decline.Int J Geriatr Psychiatry. 2015; 30: 422-430Crossref PubMed Google Scholar, 3Hamer M Stamatakis E Prospective study of sedentary behavior, risk of depression, and cognitive impairment.Med Sci Sports Exerc. 2014; 46: 718-723Crossref PubMed Scopus (168) Google Scholar whereas others (eg, watching television) have been associated with worse cognitive performance.3Hamer M Stamatakis E Prospective study of sedentary behavior, risk of depression, and cognitive impairment.Med Sci Sports Exerc. 2014; 46: 718-723Crossref PubMed Scopus (168) Google Scholar As such, although not the focus of our Article, the role of sedentary behaviour in cognitive ageing poses many areas to explore in future work. The question of adverse childhood experiences is also interesting. A previous study using the English Longitudinal Study of Ageing examined associations between adverse childhood experiences and memory performance and decline, and found adverse childhood experiences were only marginally associated with memory performance and were not associated with 10-year memory decline.4O'Shea BQ Demakakos P Cadar D Kobayashi LC Adverse childhood experiences and rate of memory decline from mid to later life: evidence from the English Longitudinal Study of Ageing.Am J Epidemiol. 2021; 190: 1294-1305Crossref Scopus (9) Google Scholar Furthermore, little evidence exists to suggest that adverse childhood experiences affect interactions between sleep and physical activity, which were the primary focus of our study. As such, not including adverse childhood experiences is unlikely to have substantively affected our results, but is a worthy consideration for future research. Finally, we agree that examining effect modification by APOE status is an interesting next step and will require a study population of sufficient size to examine the three-way interactions between sleep, physical activity, and APOE with precision. Author declarations remain the same as in the original Article.